Digital printing inks, particularly inkjet and aerosol jet inks, can be applied to a variety of substrates to fulfill different market needs. In some markets, such as graphics for banners and lorry sides, the inks are required to be very flexible, or cracking can result when the substrate is deformed. Traditional UV inkjet inks based on multifunctional monomers meet the technical needs of fast cure and good resistance properties, but are typically far too low in their flexibility and generally have poor adhesion properties to plastics, and so easily result in cracking and ink removal.
The use of very flexible inks based on monofunctional monomers is fundamentally unsatisfactory because of the very low cure speeds and the very low hardness and resistance properties that result.
Many digital printing ink manufacturers have overcome this limitation of monofunctional only inks by combining the monofunctional monomer with a material such as N-vinyl caprolactam or N-vinyl pyrollidone (EP2399965, US2012/0026235). Satisfactory properties can also be achieved by further using a mixture of monofunctional and multifunctional monomers, as defined in WO2005/026270.
However, such approaches using monofunctional monomer technology have the limitation that the resultant inks are inherently thermoplastic and still prone to blocking or set-off in a stack in the most demanding applications.
It is well known by those skilled in the art in the field of UV inkjet inks that the UV monomer isobornyl acrylate is particularly useful in combating this effect as it has a high glass transition temperature (Tg) when cured, making it appear particularly hard and scratch resistant (US2012/0026235). However, these useful properties are countered by a well-known strong odor, often precluding its use. Similar hard acrylate monomer structures useful in UV monofunctional type ink-jet inks are defined in US2009/0087576, although the levels here are preferably above 40 wt % in the formulation to achieve satisfactory results.
Alternatively, EP2399965 describes the use of a mixture comprising 10-35% of a cyclic monofunctional acrylate where the homopolymer has a Tg>20° C., 10-30% of an N-vinyl lactam and/or a vinyl ether acrylate, and 10-30% of an ethoxylated and/or propoxylated polyacrylate having a molecular weight of at least 450. In this case, the cyclic monofunctional acrylate is preferred to be 4-tert.butylcyclohexylacrylate or isobornyl acrylate.
What is clear is that there is no suitable approach to producing fast curing inkjet inks which have good adhesion, flexibility and particularly a high surface hardness which prevents them from blocking in a stack or causing ink color to be transferred to the reverse of an adjacent sheet in a stack or to the reverse of the next layer within a roll.
What the present invention discloses, which is surprising and unobvious, is that the use of the acrylate monomer as defined in Formula (I) or Formula (A) below, and in particular o-phenyl phenoxyethyl acrylate, which in itself does not have a particularly high Tg value, as a component in UV ink jet compositions, leads to an ink which has very high hardness and blocking resistance. These properties are superior even to isobornyl acrylate, with the added advantages of substantially reduced odor. Other key properties of the cured inks such as adhesion and flexibility are also maintained using this material.
As discussed below, the prior art does not disclose the use of a material of formula (A) or formula (I) as a stand-alone component of printing ink compositions.
For instance, JP2011178981 discloses inkjet recording ink compositions capable of providing high quality, glossy recorded matters on recording media with low ink absorption. Ethoxylated phenyl phenoxyethyl acrylate is used but only as part of a copolymerization resin and not in its free form.
EP2343345 discloses ink compositions for inkjet recording that can provide a high-quality recorded matter on various recording media, especially, even on a recording medium having a low ink-absorbing property, such as synthetic paper printing or offset printing paper. The inkjet compositions disclosed contain an ethoxylated phenyl phenoxyethyl acrylate as part of a resin and not in its monofunctional acrylate monomer state.
US2010001021 discloses ink compositions with improved storage stability and anticlogging properties. These ink compositions contain an oxyethyl acrylate resin which is a resin that contains approximately 75% by weight of a monomer having an oxyethyl acrylate structure represented by CAS No. 72009-86-0 and that has a molecular weight of 6900. Accordingly, the ethoxylated phenyl phenoxyethyl acrylate monomer is part of an acrylic copolymer and not used in its free form.